1. Field of the Invention
This invention relates to light sensing instruments.
More particularly, the present invention relates to means and method for sensing and measuring luminescence and reflectance.
In a further and more specific aspect, the instant invention concerns a portable device for sensing and measuring luminescence and reflectance of selected targets in the presence of sunlight.
2. Prior Art
It is generally well recognized that a ray or line of light is actually a continuously moving stream of energy particles termed "photons". The photons are emitted from the light source in pulses. Traveling at incomparable speed and being almost immeasurably diminutive, a stream of photons assumes wave-like characteristics.
Analogous to other wave forms, light has the properties of speed, frequency and wavelength. The speed is a constant, being the speed of light. Both frequency and wavelength are variable. Accordingly, properties for any type of light can be delineated by the formula: EQU c=f.times.(.lambda.)
where:
c=speed of light PA1 f=frequency; and PA1 .lambda.=wavelength.
Types of light are generally referenced with respect to the corresponding wavelength. The known types of light are juxtaposed along a continuum ranging from the short gamma rays, having wavelengths in the range of 1.times.10.sup.-4 Angstroms, to the long radio waves, having wavelengths in the range of 1.times.10.sup.17 .ANG.. Apparatus for producing light within a specific narrow band, such as an X-ray machine, are well known. The sun emits the full spectrum of light.
However, it is now recognized that light from the sun is not uniformly intense along the wavelength gradient. Throughout the spectrum are instances of the absence or diminishing of light, causing a dip in a plot of spectral energy against frequency. A number of these dips are known as Fraunhofer Lines, and numerous Fraunhofer Lines, or absorption bands, can be found along the light spectrum. As is well known in the art, Fraunhofer Lines result from selective absorption of narrow light frequencies by gases surrounding the sun.
Light falling upon a body is either reflected or absorbed. Photons striking a surface and not absorbed, leave the surface at a substantially identical wavelength. This photon behavior is ordinarily called "reflection". Reflected light emulates the source light. Thus, in the case of reflected sunlight, the Fraunhofer absorption bands are present.
The behavior of a photon being absorbed by material and causing the reemittance of light is referred to as "luminescence". Luminescent light is at another, usually longer wavelength than the excitation source light and does not contain the Fraunhofer Lines or dips. Solar-stimulated luminescence is a naturally occurring phenomenon in various sources, such as mineral deposits and vegetation.
It is well known that insight into nature and composition of a luminescent substance can be achieved by inspection of the emanant light. This is readily accomplished by employing a spectrophotometer under laboratory conditions. Field exploration for luminescence materials has been carried out in the past on dark nights by using ultraviolet lamps to stimulate luminescence and the human eye as the detector. The severe limitations of such nighttime field efforts are notoriously well known to exploration geologists.
Although sunlight excites and stimulates luminescence in a substance upon which it shines, sunlight simultaneously masks the relatively faint luminescence of the substance with a large energy return from reflectance. Thus, sensing solar stimulated luminescence is a formidable undertaking which, however, may be accomplished by taking advantage of the presence of the previously mentioned Fraunhofer Lines in the spectrum of sunlight impinging on the object being observed.
Sunlight generally shows a very sharp Fraunhofer Line in a measurement of light intensity, whereas a luminescent substance shows no Fraunhofer Line in its light intensity in the same spectrum range. Yet, the combination of the luminescent radiation of the substance and reflected sunlight will yield a measurement of intensity of a level nearly equal to that of direct sunlight with a greatly reduced Fraunhofer Line. Individual substances radiate in differing amounts, thereby reducing Fraunhofer Line of reflected sunlight in difference degrees. Charts of the various luminescent radiations of different substances are readily available or may be experimentally determined. Accordingly, by measuring the intensity of direct sunlight within a given waveband and its corresponding Fraunhofer Line, and comparing it to the intensity of reflected sunlight from a luminescent target with its altered Fraunhofer Line within the same waveband, it is possible to calculate the change in the dip attributable to the luminescent radiation of the target and hence the luminescence. Subsequent comparision to a chart of known values will identify or give insight into the target substance.
A device for this purpose is set forth in U.S. Pat. No. 3,598,994 upon which is based the famous Fraunhofer Line Discriminator used for some years by the U.S. Geological Survey at Flagstaff, Arizona. The subject device simultaneously takes a reading of direct sunlight within a narrow waveband and its spectrally corresponding immediately adjacent Fraunhofer Line, and a reading of reflected sunlight and luminescent radiation of a substance and that corresponding Fraunhofer Line within the same waveband as the reading for direct sunlight.
The prior art device, requiring simultaneous readings of direct sunlight and reflected light, necessitates a plurality of lenses, filters and prism in an arrangement requiring an inordinate amount of space, making it impossible for a user to carry the unit in one's hands. Additionally, the number and type of lenses and prisms make the device very heavy and excessively expensive to produce. Further, the Fabry-Perot type filter, as used in the prior art device, operate properly only within an exceedingly narrow temperature range, thereby mandating an adjunct temperature control unit adding materially to the weight, bulk and cumbersomeness. It is also noted that the device is not suitable for rapid, convenient adaptation for operation in multiple selected wavebands.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide improved means and method for sensing and measuring luminescence emanating from a selected target.
Another object of the invention is the provision of luminescence sensor of substantially reduced weight and bulk rendering the sensor hand portable.
And another object of this invention is to provide a luminescence sensor which is relatively insensitive to temperature deviations within a range as normally occuring throughout a typical day.
Yet another object of the invention is the provision of a portable luminescence sensor having a readily changeable optical assembly to accommodate a selected luminescent target.
Still a further object of the invention is the provision of a portable luminescence sensor which is comparatively simple and is inexpensive to fabricate.
Yet a further object of this invention is to provide a portable luminescence sensor which is relatively unencumbered and substantially maintenance free.
And a further object of the invention is the provision of a device of the foregoing character which may be assembled in a manually portable package.